The invention relates to a linear drive, especially for a belt tensioner.
Linear drives in which a driven piston covers a predetermined travel distance are used as mechanical drive elements. The linear movement can be directly used for a push or pull movement, for example to withdraw a belt buckle. However, it can also be converted into a rotating movement, for example by means of a gear rack, so as to rotate, for instance, the belt reel of a belt retractor.
The kinetic energy is generated by combusting a pyrotechnical propellant and by the expansion of the hot gas being produced. The release of the gas has to be adapted to the parameters defining the movement of the moved piston, such as the housing size and the mass, so as to obtain an optimum acceleration of the piston and an optimum force transmission. It has turned out that it is favorable to have part of the gas flow off the combustion chamber after the beginning of movement of the component part.
It is known in this context, for instance, to provide one or more radial openings connecting the piston seat to the environment in the housing of the linear drive in the wall of a piston seat close to the pyrotechnical propellant moving the piston. Said openings are released shortly after the start of shifting the piston so that part of the gas can flow off the piston seat. In this way, on the one hand a motion-dependent pressure control and a well-directed pressure decrease after completed movement of the piston can take place.
Such linear drive is shown in FIG. 1. In a housing 10 a piston seat 12 and a cavity 14 directly connected thereto are formed. A piston element 16 is arranged in the piston seat 12, viz. before the beginning of the displacing movement in the displacing direction R directly at the left end of the piston seat 12 in FIG. 1 in direct vicinity to a pyrotechnical igniter 18. The piston element 16 includes a seat 20 in which a pyrotechnical propellant (not shown) is comprised that supplies the major part of the drive gas when ignited by the igniter 18.
The piston element 16 is rigidly connected to a gear rack 22 extending in parallel to the piston element 16 and being connected to the same at the end of the piston element 16 facing away from the igniter 18. When the piston element 16 moves in the displacing direction R, the teeth of the gear rack 22 mesh with the teeth of a gearwheel 24 connected to a belt reel not shown in detail and start to rotate the same.
In the wall of the housing 10 two small radial bores 26 are formed which lead from the piston seat 12 to the housing portion in which the gear rack 22 is accommodated and from there into the environment of the housing 10.
Before the displacing movement of the piston element 16 starts, the seat 20 is sealed inside the piston element 16 and the piston seat 12 is sealed against the bores 26 by a sealing member 28, in this case an O-ring, disposed at the igniter-side end of the piston element 16. Only when the sealing member 28 has passed the bores 26 upon movement of the piston element 16, gas flows off the piston seat 12.
Producing the radial bores 26 requires a relatively complicated operating step, which extends the production time and thus renders the linear drive more expensive.